Lead-Producer Jeff Gardiner stated that he played Fallout 4 for over 400 hours and stil didn’t see everything there is.
Of course the new crafting system plays a big role in that because better items will need a lot of different ressources which will need a lot of time to find.
An example for that are the electronical devices the player can craft. They give you much control and are fun but first you will have to make clear you have a source of electricity and stuff like that.
But despite all that it is pretty obvious that Fallout 4 will be a huge game with a lot to do and to explore, not only new craftable items.